1. Field of the Invention
The present invention relates to a measuring apparatus for measuring the rotational angular position of a rotary table and a displacement in a direction along its rotational center axis, and an accuracy analyzing apparatus, equipped with the measuring apparatus, for analyzing rotational accuracy of the rotary table, accuracy of its rotation operation, and dynamic accuracy in the direction along its rotational center axis.
2. Description of the Prior Art
In the field of machine tools, for example, a rotary table is used that is suitably equipped with a horizontal loading surface, and that can be indexed at a prescribed rotational angle (rotational angular position) by rotating about its rotational center axis extending at right angles to the loading surface; since the indexing accuracy of such a rotary table and the dynamic accuracy in the direction along its rotational center axis directly affect the machining accuracy, these accuracies are suitably measured as needed at the time of use as well as at the time of manufacture.
Here, the dynamic accuracy in the direction along the rotational center axis of the rotary table concerns the displacement of the loading surface that occurs in the direction along the rotational center axis (i.e., in the vertical direction) when the rotary table is rotated about its rotational center axis. This displacement occurs, for example, when the rotational center axis runs out during rotation of the rotary table, or when the loading surface of the rotary table is not properly oriented at right angles to the rotational center axis, but is tilted from the horizontal.
In the prior art, no apparatus has been known that can comprehensively measure the indexing accuracy of such a rotary table and the accuracy in the direction along its rotational center axis, and therefore, these accuracies have been measured individually by using different apparatuses, as briefly described below.
First, the accuracy in the direction along the rotational center axis of the rotary table has been measured using, for example, a level or like instrument, as a tilt of the upper surface of the rotary table against the horizontal plane. However, the measurement using such a level is of static nature and is limited to the case where the loading surface of the rotary table is tilted with respect to the horizontal plane; therefore, when the rotational center axis is running out, the run-out cannot be detected.
On the other hand, the indexing accuracy is measured using an autocollimator, but in this case, there is a limit to the number of reflecting mirrors that can be used, and generally it is only possible to measure the indexing accuracy every 30 degrees. In view of this, measurements have been attempted using rotary encoders that can make measurements at any given rotational angle.
There are various types of rotary encoders, such as optical type, magnetic type, contact type, and capacitance type, but the basic structure is common, that is, the rotary encoder includes two members, the first and second members, mounted in relatively rotatable fashion about a preset rotational center axis, one member being provided with a scale and the other with a reading device; here, the first member is, for example, fixed to the loading surface of the rotary table and the second member is held in a non-rotating condition, and in this condition, the scale position is detected by the reading device to detect the relative rotational angular position between the two members, thus measuring the indexing accuracy of the rotary table.
Here, the relative positional relationship between the scale and the reading device is extremely important in detecting the relative rotational angular position between the two members, and if the positional relationship between the two is displaced from the initially set state, the rotational angular position cannot be measured accurately. Accordingly, when measuring the indexing accuracy of the rotary table by using such a rotary encoder, the rotational center axis of the two members constituting the rotary encoder must be accurately aligned with the rotational center axis of the rotary table.
However, in the prior art, there has not been available any apparatus that can align the rotational center axis of the rotary encoder with the rotational center axis of the rotary table in a simple manner, and there has been no choice but to employ, for example, a technique in which, with a contact type indicator held in contact with the outer periphery of the rotary encoder on the rotary table, the rotary table is rotated and, while measuring the relative displacement between the rotational center axis of the rotary encoder and the rotational center axis of the rotary table, the position of the rotary encoder on the rotary table is adjusted manually so that the measured displacement becomes zero.
However, manually aligning the rotational center axis of the rotary encoder with the rotational center axis of the rotary table is extremely inefficient as the manual adjustment is not only time consuming but requires skill.
The present invention has been devised in view of the above situation, and it is an object of the invention to provide a measuring apparatus that can easily and accurately measure the rotational angular position of the rotary table and the displacement in the direction along its rotational center axis, and an accuracy analyzing apparatus that can easily and accurately analyze the rotational accuracy of the rotary table, the accuracy of its rotation operation, and the dynamic accuracy in the direction along its rotational center axis.